Given that you have to hold records for ~7 years (last I checked), and given the recent (2016/2017) focus shift from the ATO to specifically look at tradies, it's a really bad gamble to do that sort of thing. The tradie gains pretty much nothing from the transaction and then has to explain away a disparity in stock acquisition claimed against the quarterly GST. If the ATO decides to audit them they will be screwed. Unless you've got a hell of a setup you're not going to be able to convincingly (legally) smooth out the disparity in the flow of money; so you either create a paperwork flaw, or you sit on a pile of cash you can't really use.
I get a lot of people offering me cash-on-the-side for jobs to try "help me out", the thought is lovely, but at the end of the day it's a lot easier to sleep knowing that your books are genuine and will stand up to scrutiny. It only takes one disgruntled customer to lodge a report/complaint to the ATO and they'll be on you.